Nine out of ten hospital wards may be at risk of overheating, increasing the dangers for vulnerable patients who are left sweltering in temperatures of more than 26C (78.8F), government advisers have warned.

Poor ventilation, thin walls, low ceilings and big windows that can barely be opened are contributing to temperatures far exceeding acceptable levels during hot weather, according to the Committee on Climate Change.

One fifth of domestic properties could also already be overheating, with flats especially vulnerable, it finds.

The number of people dying prematurely from overheating could triple to 7,000 per year by the 2050s as global warming makes deadly heatwaves of the kind seen in 2003 the norm, unless housing and hospitals are adapted to cope with the heat, the CCC says.

In a wide-ranging report chaired by Lord Krebs, the CCC – the government’s official climate change advisors – suggest England is ill-prepared for the increased heatwaves and flooding that are expected to arise from “inevitable” global warming in coming decades.

The committee warns: “Exposure to heat is already an issue for health. Types of hospital ward that are vulnerable to overheating currently make up 90pc of the total stock [by floorspace].

“Overheating in hospitals is a serious issue given the vulnerability of patients.”

The assessment of hospital wards comes from forthcoming Cambridge University research, which shows that temperatures are already exceeding 30C (86F) in some wards when the outside temperature is 22C (71.6F) – suggesting many hospital wards may have reached uncomfortably high temperatures during last week’s warm weather.

It finds that some £17.5bn of upgrades could be needed to make hospitals resilient to the heat through measures such as tinted windows and improved ventilation.

Monitoring of several hospital types showed temperatures breaching 26C - the threshold at which sleep is interrupted - and 28C - the threshold above which it is deemed to be uncomfortably hot.

Professor Alan Short, the author of the Cambridge research, said that most modern hospitals were all of a type prone to overheating, with the worst performers being “very lightweight 1960s buildings”, with lots of windows, often pointing southwards, that were restrained from opening, high occupancy and thin walls.

Health and safety rules “not to allow any windows to be opened by more than four inches” had the “unintended consequence” of preventing adequate ventilation, he said.

The only type of hospital identified as being resilient to overheating were “Nightingale ward” buildings, which the last Labour government vowed to eliminate because they involved large numbers of patients on mixed wards.

Prof Short and Lord Krebs both said they did not advocate widespread installation of air conditioning because it would use large amounts of energy and contribute to climate change.

Katherine Murphy, chief executive of the Patients Association, said: "Patients deserve to be treated in a safe, clean and comfortable environment. There is no excuse for patients having to tolerate hot and stuffy wards without proper ventilation. Wards that are too hot can have a huge impact on the wellbeing of patients and the staff who treat them.

"Whilst we acknowledge that many of our hospital buildings are old, there must be mechanisms in place to control temperatures. It is important that there is proper investment and maintenance of our hospital buildings in order to bring them up to standard and also to prevent unnecessary energy costs."

A Department of Health spokesman said: “The annual Heatwave Plan for England recognises the importance of long-term planning. We have also issued guidance this year on how to protect NHS buildings and ensure they are resilient to climate change.”